Cardin’s campaign announced Mfume’s support Friday. Mfume, the former president and CEO of the National Association for the Advancement of Colored People, called Cardin a “principled leader who understands that compromise is possible” in Washington.

On Thursday, State Sen. C. Anthony Muse announced his plans to challenge Cardin in the primary. Muse has been a state senator since 2007.

The Cardin campaign says it also has support from Gov. Martin O’Malley, Lt. Gov. Anthony Brown, Baltimore Mayor Stephanie Rawlings-Blake and county executives from Montgomery, Prince George’s, Howard and Baltimore counties.